Patients with cancer or trauma within the extremities often suffer chronic pain that may be difficult to control without special techniques. One such technique is peripheral nerve stimulation either percutaneously or by implantation of electronic stimulating devices. Several mechanisms have been proposed to explain the beneficial analgesic effect produced from peripheral nerve excitation. There is conflicting evidence as to whether or not there may be intrinsic excitability changes within peripheral nerves which may account for the analgesic phenomenon. We propose to clarify this important issue and establish more precisely, the kinds of sensory phenomena that can be associated with identifiable nerve fiber components. Patients that have been identified as excellent neurosurgical candidates for chronic peripheral nerve stimulation (and who have given full consent for the test procedures) will have the appropriate nerve(s) exposed under local anesthesia. Electrophysiological activity evoked in peripheral nerve and cerebral cortex will be recorded in response to stimulation of the nerve. The following factors will be rigorously measured: (1) nerve threshold, (2) the analgesic threshold, (3) average cortical response threshold, (4) sensory detection threshold, (5) peripheral nerve and cerebral cortical activity associated with the previous threshold phenomena, and (6) identification of peripheral nerve components associated with modification of pain perception. The results should not only lead to improvement in the therapeutic use of peripheral nerve stimulation for relief of chronic pain, but also clarify the mechanism by which such stimulation induces analgesia.